In conventional electric motors, the components of the motor are rigid and are in direct contact. These rigid parts are typically metal. Having these rigid parts in direct contact is desirable in some respects. For example, the direct contact of metal parts facilitates heat transfer and allows the motor to cool sufficiently during operation. In addition, the direct coupling of rigid parts helps to keep moving parts in proper alignment. However, the direct contact of rigid parts also facilitates the transmission of vibrations and consequently results in significant levels of noise.
For example, in conventional brushless direct current ("DC") motor construction (FIG. 2), the bearings are fastened directly to the rotor shaft and the motor housing. The stator stack is also connected directly to the motor housing. The shaft of the rotor is typically made of steel. The bearings are also typically made of steel. The housing is typically aluminum or steel.
At rotational speed there will be some degree of residual rotor unbalance, even if a pre-balancing operation is performed prior to use. This unbalance causes vibration in the front and rear bearings. The vibration is conducted, through the metal-to-metal fit of the shaft bearing and housing, directly to the mounting surface. This vibration of the mounting surface will be transmitted to the machine that contains the motor, creating noise in the frame or other components of the machine.
Any vibration that is generated in the rotor is transmitted to the motor stator by variation in the motor air gap. This variation in air gap changes the radial forces on the stator because of the change in magnetic attraction between the stator and rotor. Stator vibration is then transmitted to the motor housing and then to the frame of the machine to which the motor is fastened.
Additional vibration in the stator is caused by magnetostriction in the stator laminations. When the motor is in operation, these laminations are constantly being stressed from round to oval as each stator pole pair is electrically energized. The magnetic stress caused by the current flowing through successive coils distorts the stack as shown in FIGS. 3A, 3B and 3C. This variation in stack stress moves the motor housing, thereby generating audible noise as in an audio speaker.
The inventor has previously used urethane bushings around the rotor bearings to dampen noise from the bearings. For example, Model No. D1H18-23-004Z sold by BEI, Kimco Magnetics Division included such urethane bushings. However, the use of urethane bushings around the rotor bearings has met with limited success in that it did not result in a significant reduction in vibration or noise.